This invention.relates to an improved method for mounting surface-mount technology (SMT) devices such as solder ball-grid array (BGA) or solder column grid array (CGA) devices onto a printed circuit board.
Among the variety of SMT attach methods available in the electronic packaging industry, BGA and CGA devices are often used for attaching devices with high I/O requirements. In these SMT device attach systems, an array of solder balls or solder columns form both the electrical and structural interconnections between the SMT device and the printed circuit (PC) board.
In a BGA attach system, the PC board is provided with one or more BGA device bonding sites and each site is provided with an array of bonding pads that matches the array of solder ball interconnections on the BGA device. To attach the BGA device onto the PC board, low-temperature solder paste, whose reflow temperature is lower than that of the BGA solder balls, is deposited onto each of the bonding pads on the PC board. Solder paste made from eutectic Pbxe2x80x94Sn solder is typically used for this application. The BGA device is then aligned and placed on a device bonding site so that the array of BGA solder balls is aligned with the array of bonding pads on the PC board. The whole assembly is then heated to a temperature sufficiently high to melt the low temperature solder paste and form metallurgical bonds between the BGA solder balls and the bonding pads. Because the solder paste has a lower melting point than the BGA solder balls, only the solder paste melts during the BGA bonding process step.
In addition to the bonding pads, a PC board typically has surface-wiring structures that provide electrical connections between various points on the surface of the PC board. Many of the bonding pads are connected to other bonding pads within the same surface-mount device bonding site or to various other points on the surface of the PC board through these surface-wiring structures. For example, FIG. 1 illustrates a conventional BGA device assembly system. In FIG. 1, a prior art PC board 20 is shown with a BGA device bonding site 25. BGA device bonding site 25 has an array of bonding pads 23 configured to match the array of solder balls 24 on BGA device 26. And as discussed above, some of the bonding pads are shown as being connected to surface-wiring structures 22a and 22b. Two types of surface-wiring structures 22a and 22b are shown for illustrative purposes. Surface-wiring structure 22a connects those bonding pads to other parts of the PC board and surface-wiring structure 22b connects two bonding pads.
Bonding pads 23 and surface-wiring structures 22a and 22b are typically formed from the PC board""s surface metallurgy generally comprising a layer of copper (Cu) base conductor layer deposited on the PC board""s surface. The Cu base conductor layer is deposited in patterns forming bonding pads 23 and surface-wiring structures 22a and 22b. Then very thin layers of nickel (Ni) and gold (Au) are deposited over the Cu at bonding pads 23. The remaining Cu base conductor layer portions without any additional metal layers form surface-wiring structures 22a and 22b. The metal layers are typically deposited using processes such as electrolytic plating or electro-less plating that are well-known in the electronics industry.
The layer of Ni plated on top of the Cu layer at bonding pads 23 functions as an adhesion promoting layer between the Cu and Au layers and also as a diffusion barrier between the Cu and Au layers. The Au layer provides corrosion protection.
The BGA device bonding site is then coated with a layer of solder mask 28. Solder mask 28 is typically a layer of organic material, such as polyimide and covers the whole BGA device bonding site but has apertures exposing the Au surface of the bonding pads 23 to allow the BGA solder balls to bond to them. Because solder does not wet to the solder mask material, the solder mask functions as a dam around each bonding pad and prevents the molten solder from bleeding out along the surface-wiring structures during the BGA bonding process. And because of its electrically insulating property, solder mask 28 also functions as a passivation layer protecting the surface-wiring structures 22a and 22b. 
FIGS. 2 and 3 illustrate the details of a conventional BGA assembly structure using a solder mask. FIG. 2 illustrates a cross-sectional view of BGA bonding site 25 from FIG. 1 before the BGA device is bonded to the PC board. The Cu/Ni/Au multi-layer structure of bonding pads 23 is shown by the reference numbers 34, 32, 30 respectively. As discussed above, surface-wiring structure 22 consists of the Cu layer only and does not have Ni or Au plated over it.
The BGA bonding site is coated with a solder mask layer 28 that has apertures 37. Apertures 37 expose Au layer 30 of the bonding pads for bonding to BGA solder balls. To bond a BGA device to BGA bonding site 25, low-temperature solder paste 29 is first deposited onto the bonding pads using typical solder paste deposition techniques used in the industry. Some examples are an off-contact printing process, such as screen printing, or an on-contact printing process, such as stenciling.
After the solder paste has been deposited, a BGA device is aligned and placed onto the bonding site so that BGA solder balls 24 align with corresponding bonding pads. The whole assembly is then heated to an elevated temperature sufficiently high to melt the low temperature solder paste. Upon cooling, the low temperature solder forms metallurgical bonds with BGA solder balls 24 and the bonding pads. PC board 20 is also shown with a multi-layer internal structure with internal wiring metallurgy 27 such as is found in many PC boards.
FIG. 3 illustrates the cross-sectional view of PC board 20 after BGA device 26 has been bonded to BGA bonding site 25. Solder balls 24 are bonded to bonding pads 23 and form the interconnections between BGA device 26 and PC board 20.
A problem with this conventional BGA attach system is that because the solder masks are formed from materials with relatively high coefficients of thermal expansion (CTE) compared to the CTE of the PC board, the mismatch of CTEs between the solder mask and the PC board causes the PC board to warp after the assembly is exposed to elevated temperature during the BGA bonding process. For example, the CTE of a typical polyimide solder mask is about 500xc3x9710xe2x88x927/xc2x0 C. compared to the CTE of Fr-4, a typical PC board material, which is about 158xc3x9710xe2x88x927/xc2x0 C. Because the solder mask has higher CTE, it expands more than the PC board. But, because the BGA bonding process temperature is above the glass transition temperature (Tg) of the solder mask, there is no stress transmitted to the PC board during the high temperature stage of the bonding process. However, upon cooling down to the room temperature, below the Tg of the solder mask, the solder mask and the PC board shrink at different rates, causing the PC board to warp.
This poses a significant reliability concern. In many electronic devices, the component assemblies undergo many thermal cycles as the devices are turned on and off during their use. These thermal cycles strain the solder interconnections because the solder mask and the PC board expand and contract different amounts and eventually may result in metal fatigue failure of the solder interconnections.
An additional problem associated with the use of a conventional solder mask is that the interface between the solder mask and the Au plated surface-wiring is subject to degradation. The degradation may be in the form of de-lamination or peeling.
The invention provides a PC board with at least one SMT device bonding site for mounting SMT devices comprising: at least one surface for mounting one or more SMT devices; at least one base conductor layer plated on a portion of the surface; at least one plated Ni outer layer deposited on the base conductor layer; and an array of bonding pads formed by selectively plating at least one layer of Au on a portion of the plated Ni outer layer, and leaving a remainder of the plated Ni outer layer exposed to form a layer of native nickel-oxide. This structure, where a plated Ni outer layer covers the base conductor layer, eliminates the need for the conventional solder mask. And because solder does not wet to the layer of native nickel-oxide on the surface of the plated Ni outer layer it serves as a solder dam preventing the molten solder balls from wetting and bleeding out beyond the bonding pads.
A typical conductor material, Cu, may be used for the base conductor layer. Alternatively the base conductor layer may have a multi-layer structure having Cu/Ni/Au layers where Au is the top layer.
The invention also includes a method of forming a PC board for mounting surface-mount devices without utilizing a conventional solder mask. The process includes providing a printed circuit board having at least one surface having at least one surface-mount device bonding site; plating at least one base conductor layer on a portion of said surface; plating at least one plated nickel outer layer on said base conductor layer; and selectively plating at least one layer of gold on a portion of said plated nickel outer layer to form an array of gold bonding pads at said surface-mount device bonding site and leaving a remainder of said plated nickel outer layer exposed to form a layer of native nickel-oxide.
It would be appreciated by one of ordinary skill in the art that the metal layers described above, such as the base conductor layer, the Ni outer layer, and the Au layer may be deposited using electrolytic plating as well as electro-less plating processes.